Traditional ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor manufacturing, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels.
There has been a continuing effort to shrink feature sizes of semiconductor devices. As semiconductor devices are scaled down in size, the depth of related P-N junctions must be reduced accordingly. Such reduced depth P-N junctions are sometimes referred to as shallow or ultra-shallow junctions. In order to form shallow or ultra-shallow junctions, it is necessary to implant dopants with low-energy ions. However, due to fundamental limitations in the extraction and transport of low-energy ions, conventional ion implantation systems may not perform satisfactorily to form shallow or ultra-shallow junctions. In response to this problem, gas cluster ion implantation has been developed to achieve shallow or ultra-shallow implants.
FIG. 1 shows a typical gas cluster ion implantation system 100. The system 100 is typically enclosed in a vacuum housing (not shown). A source gas may be introduced into the vacuum housing via a properly shaped nozzle 102. A suitable source gas may include, for example, one or more inert gases (e.g., argon), oxygen-containing gases (e.g., oxygen and carbon dioxide), nitrogen-containing gases (e.g., nitrogen), and other dopant-containing gases (e.g., diborane). The nozzle 102 may inject the source gas at a high speed (e.g., supersonic speed). Since the vacuum chamber is at a much lower pressure than the source gas, the injected source gas will experience an instant expansion that results in cooling and condensation of the injected source gas. That is, the source gas will condense into a jet 10 of gas clusters wherein each gas cluster may have a few to several thousands atoms or molecules. The cluster jet 10 may then go through a skimmer 104 that removes stray atoms or molecules from the cluster jet 10. The resulting cluster jet 12 may be ionized in an ionizer 106. The ionizer 106 typically produces thermo-electrons and causes them to collide with the gas clusters in the cluster jet 12, thereby ionizing the gas clusters to form a gas cluster ion beam 14. Each gas cluster typically has one positive charge. The gas cluster ion beam 14 may further pass through one or more sets of electrodes 108 that may focus the gas cluster ion beam 14 and/or accelerate it to a desired energy. The gas cluster ion beam 14 may also be filtered through a mass analyzer 110 that selects gas clusters of desired mass(es). For example, the mass analyzer 110 may deflect all monomer ions and other light ions and only allow more massive gas clusters to pass through. Finally, the gas cluster ion beam 14 may be directed to a wafer (not shown) which is typically housed in an end station (not shown). The wafer may be mechanically scanned and/or tilted during an implantation with the gas cluster ion beam 14. A neutralizer 112 may generate electrons to offset charge buildup on the wafer.
The adoption of gas cluster ion implantation significantly improves the performance of ultra shallow junctions. It is now possible to implant atoms to a depth of 5-100 angstroms. So far, however, gas cluster ion implantation has been limited to the use of spot beams of gas clusters. To use a single spot beam in a uniform implantation, the spot beam has to be scanned multiple times across an entire wafer, which may not be efficient for large wafers (which may be up to 300 mm in diameter these days). In addition, the use of spot beams requires a complex design of end stations in order to accommodate two-dimensional wafer movements.
In view of the foregoing, it would be desirable to provide a solution for gas cluster ion implantation which overcomes the above-described inadequacies and shortcomings.